Phase Transformation as a Function of Particle Size in Nanocrystalline Zirconia
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613 Mat. Res. Soc. Symp. Proc. Vol. 481 ©@1998 Materials Research Society
EXPERIMENTAL PROCEDURES Pure nanocrystalline ZrO 2 powder was prepared by the so-called Liquid Thermal Spray Synthesis method [12]. In this technique, a conventional atmospheric plasma spray torch with a special injector was used to produce the high temperature plasma flame. The injector, which works on the two-fluid atomization principle, produces atomized droplets of the liquid organo-metallic precursor and injects it into the high pressure plume of the plasma jet. A parallel plate electrostatic precipitator, which consists of a pair of polished stainless steel plates separated by a 10 mm thick ceramic insulator, is placed parallel to the trajectory of the spray jet. An electric field of 6 kV/cm was applied in the electrostatic precipitator (ESP) so that the spray particles could be collected on the plates by electrophoresis. The electrostatic precipitator can be replaced by just one polished steel plate (called the "substrate") which is placed perpendicular to the trajectory of the spray beam. Details of this technique are published elsewhere [12]. The collected powder was then annealed at 500 or 9000 C for 1 hour in air. The phase composition of the powder specimens was established by x-ray diffraction (Cu-Kux radiation), and XRD peak width analysis was carried out to estimate the particle size. A small quantity (0.00 1g) of the collected powder was dispersed ultrasonically in alcohol and then allowed to settle on carbon coated copper grids for investigation by transmission electron microscopy (TEM). Bright field and dark field images were used to study the particle size and size distribution, and selected area diffraction was used for phase identification RESULTS AND DISCUSSION The XRD spectra of powder collected on ESP show that only tetragonal zirconia phase is present. The average grain size of nanopowders was calculated from line broadening using the Scherrer relationship. XRD of a standard silica specimen was used to measure instrument broadening. Contributions due to Kct 1 and KaX2 were deconvoluted. The method of Klug & Alexander [13] for correction of stress effects yielded 5.6 nm as the average grain size of as-prepared powder. The grain size distribution shown in Fig. la, determined from bright-field and dark-field TEM images, is lognormal and the average grain size is 5.2±2.Onm which is in good agreement with XRD results. The particle size distribution was lognormal, as confirmed by both Kolmogorov-Smirnov and X tests at a significant level a=5%. A typical bright-field image on Fig. 2 shows single, partially overlapping, featureless, near-spherical nanocrystalline ZrO 2 particles. Particle sizes range from 2 to 20 nm. 25
25
20--
20
CO 15
m15
Ž10Z1
0
0A 2 3
4 5
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7 8 9 10 11 12 13 14 15 16 17 Crystallite size [nm]
2
Fig. 1 a) Grain size distribution of as-prepared nano-
3
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7 8 9 10 11 12 13 14 15 16 17 Crystallite size [nm]
b) Grain size distribution of tetragonal zirconia parti-
crystalline tetragonal
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